Aging is associated with the development of nonresolving pulmonary fibrosis in both human and experimental animals. Stiffening of the extracellular matrix (ECM) is a prominent feature of lung fibrosis. Interactions between stiffened fibrotic ECM and effector cells of tissue fibrosis, known as myofibroblasts (MFBs), provide a feedforward mechanism that sustains/amplifies lung fibrosis. Targeting matrix stiffness to break the profibrotic feedback loop represents a promising strategy for treatment of persistent/progressive lung fibrosis. The current study aimed to test a proof-of-concept of matrix de-stiffening therapy against lung fibrosis associated with aging. Previous studies have shown that the formation of advanced glycation end-products (AGEs) accumulates with aging and occurs at an accelerated rate in lung fibrosis. The ECM, in particular collagen matrix, is highly susceptible to glycation due to its slow turnover rate. AGEs drives nonenzymatic crosslinking of collagen fibers. The formation of intra- and inter-molecular collagen crosslinks is a crucial factor that stiffens the ECM. In preliminary studies, we targeted AGE-mediated glycation crosslinking for matrix de-stiffening and potential anti-fibrotic therapy against aging-associated nonresolving lung fibrosis in a bleomycin injury-induced mouse model. We found that anti-AGE treatment reduces the amount of crosslinked lung collagens, decreases lung stiffness, and promotes the resolution of experimental lung fibrosis in aged mice. We identified mouse double minute 4 (MDM4), a major endogenous inhibitor of p53, as a matrix stiffness-regulated mechanosensitive molecule. Reducing matrix stiffness downregulates MDM4 expression, resulting in p53 de- repression/activation. Gain of p53 function sensitizes lung MFBs to apoptosis by upregulating Fas and induces immunogenic conversion of MFBs that release CX3CL1 chemokine to recruit macrophages and express Death Domain 1? (DD1?) engulfment receptor to facilitate macrophage-mediated phagocytosis of apoptotic MFBs. These findings suggest that matrix de-stiffening by targeting nonenzymatic AGE crosslinking activates a fibrosis resolution pathway. The central hypothesis of the current project is that matrix de-stiffening sensing by MDM4 promotes lung MFB clearance and the reversal of aging-associated nonresolving pulmonary fibrosis. Specific aims are: (1) determine the mechanisms by which matrix stiffness regulates MDM4 expression; (2) determine whether matrix stiffness sensing by MDM4 mediates the clearance of lung MFBs by activation of a p53-directed gene program involving Fas, CX3CL1 and DD1?; and (3) determine the role of mechanosensitive MDM4 in the fate decisions of lung MFBs and the reversal of aging-associated pulmonary fibrosis in mice. The proposed study will elucidate cellular and molecular mechanisms involved in the reversal of aging-associated lung fibrosis by targeting nonenzymatic AGE crosslinking. The hypothesis, if proven, will establish a proof-of- concept of matrix de-stiffening therapy against aging-associated nonresolving pulmonary fibrosis.